The present invention relates to an improved stator winding configuration for alternating current machines. The winding configuration of the present invention obtains a more sinusoidal shape in the magnetomotive force and flux wave employed in the machine. This improves the speed-torque curve and efficiency of an a.c. motor. Stray load losses, heating, noise, and vibration are reduced. When employed in an a.c. generator, the winding configuration improves the output voltage wave form.
In the following, the polyphase alternating current machine is described as an a.c. motor, it being understood that the description is applicable by analogy to an a.c. generator, or alternator. An a.c. motor has a stator winding that produces a rotating magnetic field when energized by polyphase alternating current. The rotating magnetic field of the stator rotates the rotor by electrodynamic interaction between the field and current carrying conductors in the rotor. The speed of the rotor is determined by the number of poles in the stator winding and the frequency of the applied alternating current.
The stator winding consists of a plurality of turns of wire collected into coils. The two sides of the coils are placed in slots in the stator core. The stator core is typically formed of a stack of ferrous metal laminations. The stator slots open along the air gap of the machine between the rotor and the stator core. End turns lying at each end of the stator core connect the sides of the coils in the slots. The coils are electrically connected together to form the stator winding.
At a minimum the stator winding must possess at least one coil for each a.c. phase for each north-south magnetic field pole pair. Thus, for a three phase, two pole (i.e. north-south pole pair) machine, there must be at least three coils: for a three phase, four pole machine there must be at least six coils, and so on. In a practical machine, a plurality of coils are used in defining the poles of the machine rather than a single coil. The plurality of coils defining each pole are grouped together and electrically connected. Similarly in a practical machine, the most commonly encountered winding configuration is of the two layer lap wound type. In such a winding, there is usually two coil groups per pole so a two pole, three phase machine will have six groups of coils: a four pole, three phase machine will have twelve coil groups, etc.
The rotating magnetic field is produced by the rotating magnetomotive force (MMF) wave established in the motor by the ampere-turns of the coils in the stator winding when energized by polyphase alternating current. The MMF wave and magnetic field wave in the motor generally correspond in shape, subject to the saturation effects of the ferromagnetic material used to form the stator core.
The rotating MMF wave contains harmonic frequencies as well as the fundamental component. This is due in part to the discrete distribution of the individual coils of the groups in the slots of the stator core. It is also due to the combination of the fields produced by the three separate phase windings in the stator. The MMF wave is not sinusoidal in shape, but changes in shape with time from a generally triangular shape to a generally trapezoidal shape responsive to the polyphase current energization.
Some of the harmonics produced in the MMF wave form rotate in the same direction as the main or fundamental component of the wave, but at a different speed than the fundamental. Others move in the opposite direction from the fundamental component of the MMF wave.
These harmonics moving at different speeds and directions produce numerous undesirable effects in the operation of the motor. Such effects include aberrations in the speed-torque curve due to parasitic torques generated by the harmonic flux waves. Additional stray load losses appear when load current flows in the machine. Increased heating from eddy currents and other sources occurs in the motor. Greater noise and vibration is also produced.
Because of these undesirable effects, efforts have been made to reduce the harmonic content of the MMF wave. These efforts are generally directed to making the MMF wave more sinusoidal. They involve the use of chorded coils, altering the distribution of the stator coils in the stator slots, and the use of skewed stator slots.
In skewing the stator slots, the slots are moved out of alignment with the rotor shaft so as to form a slightly helical pattern on the air gap surface of the stator core. This primarily reduces higher order harmonics.
The use of chorded coils involves coils of less than full pitch. In a full pitch coil, the sides of the coil lying in the stator slots are 180 electrical degrees apart. This may perhaps best be understood in a two pole machine in whch mechanical degrees and electrical degrees are the same. At full pitch, one side of a given coil would be positioned in the stator core diametrically opposite the other side, i.e. the sides of the coils would be angularly displaced 180.degree. about the stator. With a chorded coil of less than full pitch, one side of the coil is displaced less than 180.degree. from the other side, for example, 150.degree.. Pitch is usually expressed in terms of percent, the above example illustrating a pitch of 83.3%. Or it may be expressed in terms of a pitch factor, as 0.833.
To reduce the harmonics, the pitch of the chorded coils is selected such that the pitch factor for the undesirable harmonics, such as the fifth and seventh harmonics is much less than that of the fundamental component.
However, in the case of a two pole machine, it is difficult to provide the desired pitch factor in the coils. This is due to the large, awkward coil end turns in such a machine. These raise manufacturing problems and are subject to space limitations in the motor. It may thus not be possible to obtain a preferred pitch factor in the coils and the resulting reduction of the harmonic content of the MMF wave.
Because of the constraints on pitch factor, coil distribution is left as a means for reducing harmonic content. By proper selection of the number of slots in the stator and the number of coils per group, some reduction in the harmonic losses can be obtained. However, undesirable harmonic content may still amount to as much as 20% of the fundamental component.
A further attempt in this direction is the so-called "interspersed" or "cyclic shift" winding configuration described in "Winding Alternating-Current Machines" by Michael Liwschitz-Garik and Celso Gentilini, published by Datarule Publishing Co., Inc., New Canaan, CT and in an earlier version of the same work authored by Liwschitz-Garik and published by van Nostrand and Co., 1950.
In an ordinary machine, the coils are distributed in the slots so that coils of the same group are adjacent. Each coil group typically occupies 60 electrical degrees in a three phase machine. A typical configuration for a two pole machine having four coils for the coil group of each phase would be as follows. In the following example, the cylindrical surface of the stator along the air gap has been unrolled to form a straight line. A, B, and C represent the three phases of the alternating current. The diagram below represents the coil sides in the bottom of the stator slots of a two layer lap wound stator winding. The top coil sides, not shown, are inserted in the slots, a coil pitch away, to form the top layer of coil sides in the stator slots. Since the pattern of top coil sides in the slots duplicates the pattern of the bottom coil sides, only the bottom coil sides are shown. The plus and minus designations represent the relative direction of a current in a conductor compared to the others of the same phase. Thus, if the current is flowing into a conductor marked A+, it would be flowing out of a conductor marked A-. The winding is as follows.
__________________________________________________________________________ + + + + - - - - + + + + - - - - + + + + - - - - A A A A B B B B C C C C A A A A B B B B C C C C __________________________________________________________________________
In an interspersed winding, the outer coils of the groups are moved outwardly from the center of the group, thereby effectively increasing the width of the coil group as follows.
__________________________________________________________________________ + - + + - + - - + - + + - + - - + - + + - + - - + - A C A A B A B B C B C C A C A A B A B B C B C C A C __________________________________________________________________________
By comparing the two windings shown above, it can be seen, for example, in connection with the C+ coil sides, that in the conventional configuration, all the C+ coil sides in the C+ coil group are adjacent. In the interspersed winding, the outer C+ coil sides in the coil group have been displaced outwardly one stator slot. An outer B- coil side and an outer A- coil side are placed where the outer C+ coil sides used to be so that each stator slot still contains one coil. This interspersion of the coil sides gives rise to the "interspersed" designation of the winding. The amount of undesirable MMF wave harmonic content attributable to coil distribution is less in an interspersed winding than in a winding of conventional distribution. This is due to a more sinusoidal form of MMF wave produced by an interspersed winding.